[Xen-devel] Re: Linux questions

To:	Andi Kleen <andi@xxxxxxxxxxxxxx>
Subject:	[Xen-devel] Re: Linux questions
From:	Keir Fraser <Keir.Fraser@xxxxxxxxxxxx>
Date:	Tue, 04 Dec 2007 11:56:51 +0000
Cc:	xen-devel@xxxxxxxxxxxxxxxxxxx, Jan Beulich <jbeulich@xxxxxxxxxx>, Samuel Thibault <samuel.thibault@xxxxxxxxxxxxx>
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On 4/12/07 11:44, "Andi Kleen" <andi@xxxxxxxxxxxxxx> wrote:

>> I must disagree! And I *know* that RDTSC is not a serialising instruction...
>> 
>> If we race, then there was an interrupt. Interrupt delivery is a
>> serialisation point for the interrupted instruction stream.
> 
> The synchronization relies on the RDTSC happening between the
> two sequence number checks. Otherwise you can get inconsistent state
> between RDTSC and the xtime data which might be changing asynchronously
> on another CPU. Therefore you need RDTSC barriers.

Our monotonic_clock() implementation does not reference xtime nor its
seqlock. If it did, you would be correct.

 -- Keir



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[Xen-devel] Re: Linux questions